Identification of genes that cause human tumors provides important insight into the mechanisms of tumorogenesis. Inherited malignancies provide an opportunity to identify these pathogenetic genes, since powerful positional cloning methodologies will identify the gene of interest. Because there are relatively few multigeneration fPTC kindreds, genetic linkage methodologies with positional cloning have not yet been used to investigate fPTC. The purpose of this proposal is to use linkage analysis as the first step in positional cloning of the fPTC susceptibility gene.